(1) Field of the Invention
The invention concerns a device for controlling the fuel-air ratio in the combustion of ground coal in a firing system of a coal-fired power station, with means for pneumatic conveyance of the ground coal to the burners of the coal-fired power station as well as means for supplying combustion air to the burners and/or in the combustion chamber of the coal-fired power station, wherein the coal-fired power station has at least the following devices in the flow direction of the air:
A fresh air blower for drawing in fresh air from the environment, a mill fan for conveying a portion of the drawn-in fresh air as carrier air for loading with ground coal, an air pre-heater to preheating the drawn-in fresh air and a portion of the carrier air by using the flue gas heat of the coal-fired power station, an airflow control device for controlling the amount of combustion air introduced into the combustion chamber, an airflow control device for controlling the carrier air used for the pneumatic conveyance of the ground coal as well as measuring devices for measuring the amount of combustion air introduced into the combustion chamber of the coal-fired power station and a device for metered supply of a preselected quantity of ground coal to the burners.
The invention furthermore relates a method for controlling the air-fuel ratio in the combustion of ground coal in one coal-fired power station, which has at least one of the features listed above.
(2) Description of Related Art
The control of fuel air-ratio in the combustion of ground coal in coal-fired power stations is particularly important for achieving substantially complete combustion of the supplied fuel or for maintaining a predetermined stoichiometry of the combustion process and hence for achieving high energy efficiency as well as for maintaining lower emission values. Combustion systems in coal-fired power stations therefore have, in addition to suitable devices for metered supply of a preselected amount of fuel to the burners commensurate with the load demand, control devices configured to control the amount of air supplied to the burners or the combustion chamber depending on the supplied amount of fuel. For this purpose, measuring devices are required which are capable of measuring, preferably exactly, the amount air supplied to the combustion chamber, i.e. the amount of combustion air and the amount of carrier air. Both are necessary for optimally regulating the combustion process depending on the load, wherein the amount of fuel and hence also the amount of carrier air is preselected according to the load demand and the amount of combustion air is controlled to achieve a predetermined stoichiometry of the combustion.
In practice, coal-fired power stations exist where a preselected amount of ground coal is supplied to each individual burner or to a group of burners and the amount of air supplied to this burner or this group of burners is controlled according to the preselected amount to ground coal to achieve a predetermined stoichiometry of the combustion, as well as combustion systems where only the total amount of ground coal supplied to all burners is selected and accordingly also only the total amount of air supplied to all burners of this boiler or to this boiler is controlled.
In each case, it is necessary to measure the amount of air supplied to the boiler either in relation to an individual burner or to a group of burners or to all burners of a boiler or to the entire boiler in order to realize the appropriate control(s) of the amount of combustion air and to thereby regulate the combustion process.
Furthermore, in combustion power stations with pneumatic conveyance of the ground coal to the burners, the amount of carrier air for the pneumatic coal transport is controlled. This control also requires a measurement of the amount of carrier air.
The airflow in coal-fired power stations is predominantly measured with pressure sensors using a differential pressure measurement. For this purpose, pressure sensors are installed in the pipe or conduit system carrying the combustion air as well as in the pipe or conduit system carrying the carrier air. The flow velocity in the channels can be determined based on the measured pressures and the respective amount of air can be determined from the channel geometry. Preferably, the pressure sensors are not installed directly in the channel cross-section, but are connected with the channels carrying the air via so-called impulse lines.
To improve the energy efficiency of a coal-fired power station, it is common to preheat the fresh air. Frequently, regenerative air pre-heaters are employed. In regenerative air pre-heaters, a storage mass, usually smooth or corrugated metal sheets, is alternately first heated with hot flue gas and subsequently cooled with the fresh air, thereby transferring heat from flue gas to the fresh air. This is accompanied by the introduction of fly ash particles in the fresh air. This regularly causes contamination of the pressure sensors or of the impulse lines during operation of the coal-fired power station. Continuous cleaning and maintenance are then required. Disadvantageously, the degree the contamination of the pressure sensors cannot be clearly determined based the measured differential pressures so that the measurement results have a significant risk of error in continuing operation. This causes a drift in the air volume measurement which is detectable only at high costs. This leads ultimately to an inaccurate control of the air-fuel ratio, accompanied by a diminished efficiency and increased emission of pollutants.
Measuring devices are known in the art which evaluate triboelectric effects from sequentially arranged sensors in the flow direction of the particle-laden flowing medium. Such measuring devices enable not only the determination of the flow velocity of a particle-laden flowing medium, but also the determination of the loading of the flowing medium. For example, DE 69634249 T2 describes a measurement system that can be used to determine the speed of a volume flow composed of a gas and a powder, the ratio between the gas and the powder in the volume flow and the degree of turbulence in the flowing powder suspension by evaluating triboelectric effects at two sensor elements arranged in the volume flow. Such measurement system makes it possible to determine the amount of a pneumatically transported powdery solid in a powder-laden gas flow. The measurement system can be used, for example, to control the quantity of a powdery solid supplied to a process. A method for determining the amount of coal dust pneumatically transported in a pipeline to a burner is known from U.S. Pat. No. 4,512,200 A, wherein triboelectric effects of the coal dust particles are also evaluated at sensors sequentially arranged in the flow direction to determine the amount of pneumatically transported coal dust.
The aforementioned measuring systems and methods for determining the flow velocity and the loading of particle-laden flowing media by evaluating triboelectric effects at sensors sequentially arranged in the flow direction of the particle-laden media are in principle suitable to determine the amount of a particulate solid transported in the flowing medium. These systems and methods are also applied with high reliability and precision. However, they fail for the determination of the volume flow of a gas without particle loading transported in a pipeline system.
Therefore, a method for controlling of air-fuel ratio in the combustion of ground coal in a coal-fired power station is proposed in DE 10 2008 030 650 A1, which employs a regenerative air pre-heater for preheating the fresh air, wherein the quantity of combustion airflow and of the carrier air is measured based on the evaluation of triboelectric effects at sensors arranged sequentially in the airflow in the flow direction of the air by using the correlation method, wherein between 0.1 mg and 10 mg fine-grained particles per m3 air are introduced in the airflow upstream of the sensors. Preferably, the fine-grained particles are introduced in the airflow only during the start-up phase of the coal-fired power station, i.e. only during the phase when ground coal is not burnt and therefore no fly ash particles are introduced in the fresh air by the regenerative air pre-heater. The expenditure for continuously supplying fine-grained particles during the entire operating time of a coal-fired power station is too high. The solution proposed in the DE 10 2008 030 650 A1 is hence only advantageous for a coal-fired power station with a regenerative air pre-heater for preheating the fresh air.
Moreover, it has been observed that the combustion air volume measurement and the carrier air volume measurement in a coal-fired power station based on the evaluation of triboelectric effects of triboelectrically charged particles carried in the airflow at sensors arranged sequentially in the flow direction of the air by using the correlation method can no longer be applied with sufficiently high measurement accuracy when the particle loading of the air and/or the flow velocity of the airflow fall below a critical value. The aforementioned critical values are at approximately a particle loading of 0.1 mg fly ash/m3 air or a flow velocity of 10 m/s, wherein both values are interdependent for realizing a high measurement accuracy such that the particle loading the air can be smaller than the specified 0.1 mg fly ash/m3 air, when the flow velocity the air is substantially greater than 10 m/sec and vice versa, and such that the flow velocity the air can also be less than 10 m/sec, while the particle loading of the air is greater than the aforementioned 0.1 mg fly ash/m3 air.